The present invention relates to a method and arrangement in doctoring in a paper or board machine, in which the web travels through at least one press nip, in which the press nip is formed by at least a press roll and its backing roll and in which at least one roll is doctored with the aid of at least one blade device.
Particularly in the wet end of a paper or board machine, which is generally understood to include at least the wire section and the press section, material from the wet web adheres to the rolls. The adhering material consists of, for example, fibers and fillers, which are formed of particulate material such as mineral fines. Impurities of this kind gathering on the roll result in, among other things, the web sticking to the roll surface and in runnability problems in the form of plucking, leading to an increase in web breaks and thus to production losses.
An essential part of roll maintenance and cleaning to remove the impurities collecting on them is their doctoring, for example, using special blade devices. In doctoring, the blade of the doctor is loaded in a known manner against the roll, in order to scrape off the impurities that have collected on the roll.
Several different types of doctoring devices are known, such as single and double-bladed doctors and those having different kinds of blade holder. Nowadays, for example, in critical roll positions in the press section of a paper machine, it is usual to use double doctors, in which two doctor blades of essentially the same length are attached to a single blade holder. The doctor blades are of the same length as the surfacing of the roll being doctored. The use of such a double-blade arrangement results in, among other things, better cleaning.
Though the blades are made from hard materials, uneven wear is nevertheless one of the problems arising when using doctor blades. Uneven wear causes a substantially dirtier area to form on the roll surface than at its edge areas. The area forms essentially at the roll segment directly in contact, for example, with the web travelling through the press nip, or indirectly in contact with the said web due to, for example, a felt remaining between the web and the roll.
Due to the differences in the cleaning of the said roll segments, a sharp, step-like discontinuity point, or burr, shown in FIGS. 1a and 1b depicting the state of the art, forms in the doctor blade at precisely a location corresponding to that point on the roll, with which the edges of the web are in either the said direct, or indirect contact. The detriments caused by the burr 40 can be reduced by inducing oscillation in the blade 23xe2x80x2. Though oscillation results in a less sharp formation of burrs 40, the blade 23xe2x80x2 still has a substantially weaker contact with the roll 10 being doctored than a newly-changed, unworn blade, for instance, will have, thus also leading to substantially poorer cleaning efficiency. The life of the blades 23xe2x80x2, 24xe2x80x2 may vary from as little as 2 hours to a few days. This also causes production losses, as the machine must generally be stopped to change the blades 23xe2x80x2, 24xe2x80x2.
During oscillation, the doctor blade is moved parallel to, and against the roll, for example, 2-3 times a minute, the roll velocity being 20-30 m/s. There are problems in maintaining continuous oscillation. For example, oscillation may be out of operation for even long periods due to a device fault, because its repair may need the entire machine to be stopped, which is generally not justified only to repair an oscillation device. In certain situations it may even be felt to be a nuisance to arrange the functioning oscillation that is necessary for the above reasons.
Further, especially in the threading stage, in which the edge band of the web is fed in stages through the machine, the deficient cleaning caused by the said burr in the doctor blade leads to significant problems. Especially at the wet end of the paper machine, the paper web is particularly sensitive to any impurity collecting on the roll. For example, impurities can make the web stick onto the roll, which is precisely one of the commonest problems causing difficulties in threading.
The manufacturing materials of the rolls being doctored have also developed. Nowadays, many rolls surfaced with, for example, rubber and composite materials are used, for their heat resistance among other reasons. However, these roll surfacings are much more susceptible to damage than traditional stone rolls, partly due to aforesaid burr that forms in the doctor blade.
At present, the construction of doctor blades can also be surfaced, at least over the width of the essentially dirtiest area of the roll being doctored and over which the roll is in either direct or indirect contact with the web. However, even the use of such special surfacing solutions is insufficient to effectively improve the grip of the blade against the roll being doctored. The blade still wears more rapidly over the width corresponding to the web than over the essentially cleaner edge segments of the roll.
The invention is intended to create a new type of method and arrangement in doctoring in a paper or board machine.
Accordingly a method in doctoring in a paper or board machine, in which the web travels through at least one press nip, in which the press nip is formed by at least a press roll and its backing roll and in which at least one roll is doctored by at least one doctor device, is characterized in that the press roll is doctored over essentially the width of the web. And the arrangement in doctoring in a paper or board machine, in which the web is arranged to travel through at least one press nip, which is arranged to be formed by at least one press roll and its backing roll and in which at least one of the said rolls is arranged to be doctored by means of at least one blade device, is characterized in that the doctoring of the roll with the blade device is arranged to take place over three roll segments, in which are arranged
first blade devices arranged essentially closer to the web for doctoring the said first roll segment area, which is arranged to cover essentially that area of the roll, which is in direct or indirect contact with the web arranged to travel through the nip and
second blades devices, which are arranged to doctor at least the edge segments of the roll that remain outside of the said first roll segment.
By means of the method and arrangement according to the invention, a considerably better grip of the doctor blade is maintained, despite the wear in the blade. This significantly improves, among other things, the cleaning of the roll, and achieves substantially easier threading, as well as reducing the number of so-called edge-fraying breaks.
Further, as one preferred embodiment, the oscillation of the doctor blade can even be shut off completely, or else operated for a much shorter time than in the state of the art, as there is essentially no formation of a burr caused by differences in blade wear due to the differences in cleanliness of the different segments of the roll, when the roll is doctored with the aid of the method according to the invention.
The method and arrangement according to the invention can be advantageously implemented both in known double-doctor constructions and even in simple single-blade doctors. Though the method and arrangement according to the invention are preferably applied particularly in the press section of a paper or board machine, they can also be advantageously used in other corresponding operating locations requiring doctoring.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.